In many chemical or biochemical reaction situations, it is necessary to detect the presence in a mixture of species of a particular target species. For example, in immunological assay reactions, it is necessary to be able to accurately detect the presence of, often very small, quantities of a particular antibody or other biochemical species.
Known methods for detecting a target species in a sample, particularly in biochemical reactions, often involve introducing into the sample a reagent which will react specifically with the target species. Once this reagent has combined with any target species present in the sample, the presence of the reagent-target pair is then detected in a number of known ways. For instance, the reagent may be labelled in some way (chemically or radioactively, for example) to allow for ease of its detection.
One particular case in which reliable detection methods are necessary is the detection of a particular antibody in a sample. Here, the reagent introduced into the sample is a specific antigen which binds to target antibody present in the sample. The presence of the antigen-antibody pair in the sample can then be detected.
Alternatively, a particular antigen may be the target species to be detected, in which case the reagent introduced into the sample is a specific antibody which binds to the target antigen.
A number of known detection methods make use of some characteristic of the antigen. For example, the technique of "latex agglutination" makes use of antigen-coated bodies whose increase in aggregate size following binding to the target antibody can be detected optically. Radioactive markers and fluorescent dyes have also been used to "label" the antigen and allow its detection following binding to the target antibody.
These known detection methods have undesirable characteristics. Any macro-scale technique (such as latex agglutination) inevitably requires large numbers of antigen-antibody pairs to be present in a sample to produce a detectable effect. Fluorescent and radioactive markers are difficult and expensive to detect with the required sensitivity.
These difficulties are often paralleled in other chemical and biochemical reaction situations where sensitive and reliable detection methods are needed.
The need for a reliable method for detection of a target species in a sample also arises in the monitoring of fluid flow through a system or of particle movement through a fluid system; and in the detection of fluid leaks from a system. In each case, a sample is taken from a particular location inside or outside the system and the presence of a Larger species in that sample detected, which then provides information as to which areas of the system the target has reached or, as the case may be, whether the target has been able to leak out of the system. In these cases, too, there is always a need for a simpler, more reliable and more sensitive detection method than those currently available.